1. Field of the Invention
The invention relates to corundum porcelain compositions containing a plurality of inorganic starting materials. The invention also relates to methods of preparing and using such composition.
2. Description of Related Art
The addition of small amounts of comminuted broken porcelain to porcelain compositions for producing tableware is known. In the production of porcelain tableware, the addition of milled and fired broken porcelain increases the shape stability of the shaped bodies during firing. Texture effects play virtually no role here.
While the porcelain tableware items have thin walls and a low weight, the components of technical ceramics, particularly high voltage insulators and pipes, can have a length up to about 6 meters or a weight of more than 1000 kg. Depending on their length, their compactness and their weight, considerable stresses can arise in the shaped body and in the sintered body during shaping, drying, and firing. In the production of high voltage insulators, these stresses can lead to the sheds breaking off during drying of insulator bodies and to fracture of the shank during suspended firing. In use, high voltage insulators are subjected to not only considerable electrical stresses, but also mechanical stresses. The mechanical properties of the high voltage insulators, therefore, play a critical role. In the case of tubes of great length, the shape stability of the sintered body is important in use.
The production of high voltage insulators comprises preparing a plastic corundum porcelain composition, extruding a cylindrical body using a vacuum extruder, turning, drying, and firing. Extrusion produces textures in the green bodies. Texture means the anisotropic alignment of the platelet-like clay particles and the other constituents of the composition which results from the action of pressing pressure, flow, and shear.
Distinction can be made between flow texture, screw texture, and cutting texture. Particularly the cutting texture, which is produced by the flights of the screw of an extruder, is critical for insulator production. A thin zone of pronounced texture which runs helically through the shaped body is formed. This zone, which often extends over a large area, is a weak zone which can cause the shaped body fired in a hanging position to break during firing. The flow texture causes an anisotropic shrinkage on drying in the shaped body. Since the anisometric raw material particles are differently aligned in the outer and inner parts of the shaped body, this causes different shrinkage on drying. Drying results in stresses which in extreme cases in the drying of high voltage insulators can lead to the sheds breaking off from the shank of the shaped body.
To avoid textures which are too great, a corundum porcelain composition which is suitable for high voltage insulators should contain a proportion of relatively coarse particles. Up to 5% by weight of the raw materials are customarily added as coarse particles having a particle size distribution between about 45 and 100 .mu.m, the coarse particles being composed of quartz, feldspar, and/or alumina agglomerates. Although commercial calcined aluminas have a significantly finer primary particle size, they are agglomerated as a result of the method of manufacture. The mean particle size of the corundum porcelain composition is then usually of the order of 8 .mu.m.
As constituent of the fired porcelain, quartz has both a strength-increasing action and a strength- and lifetime-reducing action owing to the .beta.-.alpha.-quartz transformation at 573.degree. C. in the cooling phase of firing, which transformation is associated with a volume contraction of the quartz grains. The glass phase is already solidified at this temperature, resulting in microstructural stresses which increase with increasing quartz grain size. Tangential compressive stresses have a strength-increasing effect in the vicinity of a quartz grain; this microstructural stressing has to be overcome before crack propagation can occur. Strength and lifetime are impaired if the compensating radial tensile stresses exceed the strength of the quartz and generate microcracks in the quartz grains. Microcracks in quartz grains can frequently be observed in porcelains on optical microscopic examination. It is known that optimum strength is achieved with quartz grains having a size of the order of from 10 to 20 .mu.m. However, raw materials able to be used on a large scale have such broad particle size distributions that proportions of relatively large, crack-prone quartz particles having a particle size up to about 80 .mu.m are also always present if these raw materials are not milled in an additional process step to particle sizes of less than 20 .mu.m for the entire particle size distribution.
Replacement of quartz by alumina (synthetic aluminum oxide and/or aluminum hydroxide powder) avoids the danger of microcrack formation since the corundum (.alpha.-Al.sub.2 O.sub.3) retained and/or formed during firing has a thermal expansion similar to that of the remaining micro-structural constituents of the corundum porcelain. Corundum is the constituent in corundum porcelain which has by far the best mechanical properties. The strength-increasing action of corundum is essentially dependent on its proportion and its grain size. It is generally known that the best strength is achieved when processing an alumina whose mean particle size after firing in the corundum porcelain is from about 3 to about 9 .mu.m. For a corundum porcelain of high strength, such a fine alumina should be added or these particle sizes should be produced by milling during preparation of the composition; the quartz should be milled as finely as possible or avoided entirely. To be sure of excluding quartz particles &gt;20 .mu.m the mean particle size of the quartz should not exceed 3 .mu.m. Even finer milling is advantageous.
Commercial feldspars too must not be added as a coarse fraction since they contain quartz as minor constituent and the feldspars, the alternatively usable feldspar substitutes or rocks containing feldspars and/or feldspar substitutes, which act as glass formers, have to be homogeneously distributed in the composition; these raw materials are, like milled broken glass and glass frits, referred to as glass formers in the following text.
The optimum strength can, therefore, only be achieved in conventional porcelain production if the starting materials are added in sufficiently fine form after prior milling or are finely milled during processing; the troublesome texture and the rejects resulting from this texture then have to be accepted. If this fine milling is omitted, an additional high proportion of the expensive alumina has to be added to achieve a sufficient strength.
In the large-scale manufacture of high voltage insulators, a substantial proportion of the rejects is caused by textures. Depending on the process procedure and insulator geometry, the rejects caused by texture are from about 1 to 3% of the components. Significant rejects resulting from textures also occur in the manufacture of other types of components from corundum porcelain. These effects are greater, the greater the degree to which shear forces occur during shaping and textures can be formed and the longer, compactor or heavier the shaped bodies are. Among the extremely varied shaping processes for ceramics, the ones which are most sensitive to the formation of textures are those which use compositions in a plastic state. These in particular include extrusion.
As indicated in the previous sections, in the production of large sintered bodies from conventional porcelain compositions a compromise has to be made according to the present state of the art between good behavior in production, particularly the reduction of textures, and high strength of the fired sintered bodies. The reduction in strength resulting from this compromise is occasionally counteracted by an additional high and expensive alumina addition, but a significant number of texture flaws nevertheless remain and these cannot be counteracted further.
EP-B-0 189 260 teaches a process for producing a high-strength, feldspar-type porcelain in which the nonplastic raw materials of the quartz, feldspar and alumina type have, corresponding to FIG. 1 of this EP document, a mean particle size of from about 2 to 4 .mu.m, the nonplastic raw materials are calcined and subsequently mixed-with the clay material. Since the calcination is carried out without clay material, no appreciable amount of mullite can be formed. There is, therefore, also an absence of the crystal nuclei which can allow a strong framework of mullite needles to be formed during firing and thus could increase the shape stability of the shaped body during firing. Before or after addition of the clay material, the calcined material is milled to a similar particle size to that possessed by the nonplastic raw materials prior to calcination. The milled mixture to which the clay material has been added is further processed by shaping, drying, and firing.
This document, therefore, teaches the repeated milling of the starting materials to an extraordinarily low particle size and the repeated carrying out of the firing. The fine-grained porcelain composition leads to a lower drying sensitivity. Owing to the fineness of this porcelain composition, it is expected that it tends to strong texture formation in the production of large bodies.